Process for preparation ores for concentration

ABSTRACT

A process for preparing ores for separation and recovery of minerals from gangue wherein crushed particles of ores are subjected to a series of sequential dry impact grinding steps in a plurality of dry centrifugal impact grinding mills to produce a product containing liberated minerals and gangue at a coarser than normal liberation size. The product can be treated by mineral separation and recovery processes to separate the minerals from gangue and recover the minerals as a usable concentrate of a predetermined grade. The efficiency of the mineral separation and recovery processes, for example, froth flotation, magnetic separation, gravity separation, and the like, is improved. The process makes the recovery of minerals more economical than prior preparation processes.

United States atent [191 Karpinski et al.

[ PROCESS FOR PREPARATION ORES FOR CONCENTRATION [73] Assignee: Bethlehem Steel Corporation,

Bethlehem, Pa.

[22] Filed: Oct. 16, 1974 [21] Appl. No.: 515,263

Primary ExaminerGranville Y. Custer, Jr.

Attorney, Agent, or FirmJoseph J. OKeefe; Charles A. Wilkinson; John S. Simitz [57] ABSTRACT A process for preparing ores for separation and recovery of minerals from gangue wherein crushed particles of ores are subjected to a series of sequential dry impact grinding steps in a plurality of dry centrifugal impact grinding mills to produce a product containing liberated minerals and gangue at a coarser than normal liberation size. The product can be treated by mineral separation and recovery processes to separate the minerals from gangue and recover the minerals as a usable concentrate of a predetermined grade. The efficiency of the mineral separation and recovery processes, for example, froth flotation, magnetic separation, gravity separation, and the like, is improved. The process makes the recovery of minerals more economical than prior preparation processes.

8 Claims, 1 Drawing Figure COMMIIVUTED ORE 7'0 RECOVERY PROCESSES PROCESS FOR PPTION ORES F OR CONCE TlON BACKGROUND OF THE INVENTION This invention is directed to a process for preparing ores for mineral separation and recovery at a relatively coarse liberation size wherein the ores are comminuted to a liberation size in a series of sequential impact grinding steps in which the minerals are liberated from gangue at a relatively coarse liberation size to produce a concentrate of a predetermined grade with the production of a minimum amount of fines.

An ore can be defined as an economically valuable mineral or an aggregate of minerals, more or less associated with gangue, which is made up of economically worthless mineral or rock material. An ore from the standpoint of the miner, can be won, or in other words mined and sold, at a profit or, from the standpoint of a metallurgist, can be treated or refined into a metallic product at a profit. The test of yielding a mineral or minerals at a profit seems, in the last analysis, to be the most feasible one to employ in defining an ore.

In recent years, high grade ores, that is, ores which contain a high percentage of minerals which can be 'recovered as a usable product requiring a minimum amount of mining and a minimum amount of preparation prior to mineral separation and recovery of the minerals from gangue, have been widely depleted. As a result, industry has had to turn to medium and low grade ores which must be processed by more sophisticated processes in order to produce an ore concentrate sufficiently rich in mineral values to make extraction of the minerals both economically and practically feasible. The medium and low grade ores are customarily prepared for concentration in mineral separation and recovery processes by initially crushing and subsequently comminuting the crushed pieces to a particle size called the liberation size. The liberation size of an ore can be defined as the size to which the ore must be comminuted in order to produce a concentrate of a predetermined grade. The liberation size varies for each ore because each ore has certain fracture characteristics determined by the physcial properties of the ore, such as grain size, hardness, brittleness, cleavage, and the like. It is virtually impossible to cormninute an ore to a particle size wherein all the minerals are liberated from associated gangue material. However, most ores can be comminuted to a particle size whereby the minerals can be economically liberated, separated and recovered from gangue to produce a concentrate of a predetermined grade. It is also economically desirable to comminute the ore to the coarsest liberation size possible which will produce the predetermined desired grade of concentrate in order to minimize the amount of grinding or comminution necessary.

Prior art methods for liberating minerals from gangue in ore and for preparing the ores for separation of the minerals from gangue and recovery of minerals from the ores, include comminuting the ore in a conventional grinding mill, such as a rod mill, a ball mill, a pebble mill, a tube mill, an autogenous mill and the like to a predetermined liberation size. These mills are normally operated in a closed circuit. In a closed grinding circuit, the oversize portion of the product from the mill is recycled back to the mill. Since the oversize can contain a high percentage of recoverable minerals, it must be recycled to the mill for regrinding to liberate the minerals from gangue. The relatively fine particles so produced include particles of liberated minerals and gangue and particles which contain both minerals and gangue. The relatively fine particles are passed to mineral separation and recovery processes, such as frothflotation, magnetic separation and the like for separation of the minerals from gangue and recovery of the minerals from the ore. It can be seen that there are certain disadvantages to the conventional closed grinding circuit in use today. Recycling the relatively coarse particles of ore as part of the feed to the grinding mill decreases the amount of new crushed ore which can be fed to the mill, resulting in decreased total throughput in the mill. The mill can become overloaded with particles of one size as sometimes occurs in autogenous mills, thereby requiring additional power input to the mill to comminute the feed to the desired relatively fine particle size. Overgrinding of the particles also occurs, resulting in the increased production of very fine particles which form slimes when mixed with water during subsequent processing. Slimes are generally detrimental to mineral separation and recovery processes. They interfere with the action of conditioning and frothing agents in the froth-flotation processes and tend to interfere with settling in gravity concentration devices. Minerals in slimes cannot be separated from gangue in gravity devices, such as jigs, tables spirals and the like, and therefore cannot be recovered as usable minerals. The prior art processes for separating and recovering minerals from ore by froth-flotation, gravity devices and the like, generally require the addition of a desliming step prior to the mineral separation and recovery processes. A substantial portion of the slimes is removed from the comminuted product in the desliming step and is passed to waste. As a result, a portion of the minerals is irretrievably lost in the wasted slimes. The desliming step is an extra step included in the process, adding to the cost of recovering the minerals in the ore. There has, therefore, been a need for a more economical process for preparing ores containing minerals, by which the minerals can be liberated from the gangue at a relatively coarse liberation size and whereby the liberated minerals can be separated and recovered in subsequent mineral separation and recovery processes without the production of a substantial amount of very fine material which may form slimes.

It is the primary object of this invention to provide a process for preparing ores for concentration, which process will alleviate the above-mentioned problems.

It is another object of this invention to provide a process for preparing ores for concentration whereby minerals are liberated from gangue at a relatively coarse liberation size and the liberated minerals can be separated and recovered from gangue by mineral processes such as froth-flotation, magnetic separation, gravity separation, and the like, to produce a concentrate of a predetermined grade with a minimum production of very fine particles.

It is another object of this invention to provide a process for preparing fluorite ores for concentration of fluorite by froth-flotation processes.

SUMMARY OF THE INVENTION The process of the present invention includes comminuting crushed particles of ores in a series of dry sequential centrifugal impact grinding steps wherein minerals are liberated from gangue at a relatively coarse liberation size with a minimum production of very fine particles. The surfaces of the liberated minerals and gangue are fresh, clean and dry. The minerals are easily separated and recovered from gangue by mineral separation and recovery processes, for example, froth flotation, magnetic concentration, gravity separation and the like, to produce a concentrate of a predetermined grade.

DESCRIPTION OF THE DRAWING The drawing is a diagrammatic representation of the process of the invention as applied to the production of a fluorite concentrate.

PREFERRED EMBODIMENT OF THE INVENTION Ores, such as fluorite, iron ores and the like, can be initially prepared by simplified conventional methods for concentration, such as crushing, screening and the like, prior to being treated in accordance with the present invention. The crushed particles of the ores are comminuted, in accordance with the invention, in a series of dry sequential centrifugal impact grinding steps to liberate the ore particles from the gangue material. The crushed particles of the ores are comminuted by hurling them against a target at a predetermined optimum velocity to obtain an energy level at impact sufficient to fracture the particles along the contact zone between the minerals and gangue. The minerals are thus liberated from the gangue at a relatively coarse liberation size with a minimum production of very fine particles.

Broadly, the dry sequential centrifugal impact grinding process of the invention which is used to liberate minerals from gangue in particles of ores, includes sizing the particles of crushed ores by means of a conventional device, for example a screen and the like. The particles of crushed ores are split into an oversize fraction containing particles of ores which do not pass through the sizing device and an undersize fraction containing particles of ores which do pass through the sizing device. The split can be made at any desired size but it is usual to make the split at about one-half of an inch or one-fourth of an inch, dependent upon the ore which is being processed. The oversize fraction has a size consist of all particles larger than the openings in the screen. The undersize fraction has a size consist containing varying amounts of particles of sizes which are smaller in size than the openings in the screen. Care must be exercized during crushing of the ores to keep the formation of the very fine particles, for example about 400mesh size and smaller, to a minimum.

The oversize fraction from the sizing device is recycled to the crushing mill to be reduced to a size which is suitable for charging into a dry centrifugal impact grinding mill. The undersize fraction is fed to a first dry centrifugal impact grinding mill which has a rotor operated at a predetermined peripheral speed which imparts a predetermined desired velocity to the coarsest size particles of the ores. The coarsest size particles are impacted at a predetermined velocity against a target. The impact energy at the time of impact is sufiicient to fracture these particles thus reducing the size of the particles and liberating the minerals from gangue along the contact zone in the particles. Since the impact of the particles against the target is a one point impact, the minerals break away from the gangue along the zone of contact, thereby liberating minerals substantially free from gangue and gangue substantially free from minerals. The predetermined peripheral speed of the rotor in the mill should usually not be sufficient to hurl medium sized particles of ores with a velocity whereby the impact energy at the time of impact against the target is sufficient to fracture the particles. The smallest particles may not be hurled with a velocity sufficient to cause the particles to strike the target. These particles are swept out of the mill before they reach the target. As a result, only the coarsest particles are comminuted, that is, reduced in size. The smaller particles remain substantially the same size, thereby the amount of very fine particles formed in the mill is reduced to a minimum. The product from the first dry centrifugal impact grinding mill can be sized at a predetermined size in conventional equipment, for example, a screen, a dry cyclone and the like, to form an oversize fraction and an undersize fraction. This sizing step can be eliminated if a very small amount of very fine particles is produced.

Either the oversize fraction, or the entire product from the first dry centrifugal impact grinding mill, is fed onto the rotor of a second dry centrifugal impact grinding mill. The rotor in the secondmill can be operated at the same predetermined peripheral speed as the rotor in the first mill or at a higher predetermined peripheral speed. The product from the second dry centrifugal impact grinding mill may be sized to form an oversize fraction and an undersize fraction or the product can be fed to the rotor of a third dry centrifugal impact grinding mill. The rotor in the third mill can be operated at the same predetermined peripheral speed as the rotor of either of the first and second dry centrifugal impact grinding mills or at a higher predetermined peripheral speed. We have found that as the particle size of the ore decreases, the rotors in the dry centrifugal impact grinding mills must be operated at successively higher peripheral speeds to impart a velocity to the particles such that the energy of the particles at the moment of impact is sufficient to fracture the particles and liberate the minerals from the gangue. We have found that if all the particles of the ores are hurled at a predetermined peripheral speed sufficient to hurl the smallest particles of the ores against the target with a velocity which will result in an impact energy sufficient to liberate minerals from gangue, all the coarse particles are virtually shattered and a product containing a large amount of very fine particles is produced. The production of a large amount of very fine particles will interfere with mineral separation and recovery processes and is to be avoided. We have found that by treating the crushed particles of ores sequentially in a plurality of dry centrifugal impact grinding steps in which the rotors of the mills are operated at predetermined peripheral speeds, an optimum amount of minerals are liberated from gangue at a coarser than normal liberation size with the production of a minimum amount of very fine particles. The minerals and gangue so produced have freshly broken, uncontaminated surfaces which react quickly and efficiently when the particles are treated in froth-flotation and magnetic separation and the like mineral separation and recovery steps to produce a concentrate of a predetermined grade.

In our process the crushed particles of ores are not reduced to any specific size, for example, where the majority of the particles are 325 mesh size or the like,

as is the case in conventional grinding circuits as conventionally used prior to treatment in mineral separation and recovery processes. The oversize between any of the dry sequential centrifugal impact grinding steps can be processed to remove barren gangue from the circuit, thus decreasing the amount of particles which must be treated in subsequent impact steps. Since the percentage of very fine particles produced is negligible, the process does not require a desliming step prior to mineral separation and recovery.

The FIGURE is a block diagram of the process of the invention as applied to the preparation of fluorite ore for concentration in which dry centrifugal air impact grinding mills were used. Vacuum impact grinding mills in which air in the mill is removed to provide at least a partial vacuum can also be used. The feed for the first dry centrifugal air impact grinding step consists essentially of particles which are /2 inch in size, that is, all the particles are less than V2 inch in size. The size consist includes small percentages of particles as small as 325 mesh sieve size. The chemical composition of the fluorite ore follows: CaF 42.2%, P 1.20%, SiO 34.9%, the remainder being CaCO A1 0 MgO and minor impurities. The rotor of the first dry centrifugal air impact grinding mill A is operated at a peripheral speed of 175 feet per second, that is, the particles of ore in the feed are hurled from the rotor of the mill with a tangential velocity of 175 feet per second. The particles are impacted against a target to liberate the minerals from the gangue. Not all the particles of ore attain the velocity required to strike the target with sufficient force to be comminuted and to liberate the minerals from the gangue. All the largest particles will be impacted and will have a portion of the fluorite ore reduced in size and fluorite will be liberated from gangue. The product from this mill consists essentially of particles having a 28 mesh size. The product is fed to the rotor of a second dry centrifugal air impact grinding mill B operating at a speed of 250 feet per second. As in the first step, the coarsest particles will be hurled from the rotor and impacted against the target whereby another portion of fluorite ore will be reduced in size and fluorite will be liberated from the gangue. The product from this mill consists essentially of particles which are 65 mesh size. The product is sized on appropriate equipment at 100 mesh size to thereby obtain a size split at 100 mesh size. The first oversize fraction, that is, all particles which are +100 mesh, are fed into a third dry centrifugal air impact grinding mill C operating at a peripheral speed of 300 feet per second. The first undersize fraction of 100 mesh size in the product from the second dry centrifugal air impact grinding mill B is passed to mineral spearation and recovery processes wherein fluorite is separated from gangue. The fluorite is recovered as a float product and the gangue, as the sink product, is passed to waste. The first processed product comprises 40.7 percent of the total weight of the fluorite ore fed to the rotor of the first dry centrifugal air impact grinding mill A. The product contains about 55.5% fluorite (CaF and 1.69% phosphorus. The fluorite contained in the product is 52.1 percent of the fluorite in the ore.

The product from the third day centrifugal air impact grinding mill C is sized on appropriate equipment to obtain a size split at 100 mesh size. The second oversize fraction, that is, all particles +100 mesh, are passed to the rotor of a fourth dry centrifugal air impact grinding mill D operating at a peripheral speed of 400 feet per second. The second undersize fraction from mill C is combined with the first undersize fraction from the second dry centrifugal air impact grinding mill and is passed to mineral separation and recovery processes as hereinafter described. The weight of the second undersize fraction is 18 percent of the total weight of ore fed to mill A and contains about 59.0% fluorite and 1.22% phosphorus. The fluorite contained in the second undersize fraction is 24.6 percent of the fluorite in the ore.

The product from the fourth dry centrifugal air impact grinding mill D is sized on appropriate equipment to obtain a size split at mesh size. The third oversize fraction, that is, all particles +70 mesh size, are substantially barren rock or gangue and are passed to waste. The third undersize fraction is combined with the first and second undersize fractions and is passed to mineral separation and recovery processes ashereinafter described. The third undersize fraction comprises about 18.3 percent of the total weight of the ore fed to the first dry centrifugal air impact grinding mill A and contains about 36.2% fluorite and about 1.08% phos phorus. The fluorite contained in the product is about 15.3 percent of the fluorite in the ore.

Note that rotors of the mills are operated at progressively higher peripheral speeds. This is necessary because as the particles become smaller during comminution they must be hurled at higher velocities to thereby be impacted against a target with a velocity sufficient to attain an impact energy of a magnitude whereby the particles will be fractured along the contact zone between the fluorite and gangue, for example silica, to liberate the fluorite from the gangue.

In each dry centrifugal air grinding step, a small portion of dust, that is, all particles which are about 400 mesh size, is formed. The dust from each of the dry centrifugal air impact grinding mills is collected and combined with the first, second and third undersize fractions and is passed to mineral separation and recovery processes as hereinafter described. The dust comprises about 4.3 percent of the total weight of the ore fed to the first dry centrifugal air impact grinding mill A and contains about 48.6% fluorite and about 0.96% phosphorus. The fluorite contained in the dusts is about 4.9 percent of the fluorite in the ore.

The total weight of material recovered in the process comprises 81.3 percent of the total original weight of ore fed to the first dry centrifugal air impact grinding mill A. About 96.9 percent of the fluorite in the original ore is recovered in the process.

The products from each of the dry centrifugal air impact grinding mills A, B, C, and D and also the dusts consist essentially of particles which have dry, clean, freshly broken surfaces. The undersize fractions and the dusts are treated in mineral separation and recovery processes as described below.

The combined undersized fractions and dusts were mixed with a depressant, quebracho. Unexpectedly, we have found that contrary to the conventional or usual practice of adding an amount of quebracho to the comminuted particles to be treated by froth-flotation at the beginning thereof and adding small amounts during subsequent froth-flotation steps to separate fluorite as a float product from gangue as a sink product, a relatively large amount of quebracho between 2.2 and 2.8 pounds per ton of dry ore fed to the froth flotation cells can be mixed with the ore feed prior to froth-flotation to take advantage of the depressant characteristics of the quebracho. It is not necessary to add any quebracho to the material in the froth-flotation cells during operation to obtain the desired separation of fluorite and gangue. Actually, we have found that the amount of quebracho added at the beginning of our process is less than the total amount which is added in prior art techniques. It is postulated that the depressant effects of quebracho are more completely taken advantage of when the quebracho is working on uncontaminated, clean, dry, freshly-broken surfaces which are formed on the fluorite and gangue in the above described preparation process rather than on contaminated and wet broken surfaces which are produced on conventional grinding processes.

Also, we have found that by grinding to a coarser liberation size, the amount of very fine particles, called slimes, formed during grinding, is reduced. Slirnes reduce the separation efficiency of depressants and collectors and consume large amounts of these reagents due to their large surface area. Hence, the reduction of the amount of slimes in the ore processed by the grinding process herein described means that a desliming step is not necessary to achieve the desired results. We also believe that the absence of a large amount of slimes is also responsible in part for the effectiveness of the quebracho mixed with the undersize fractions and dust prior to separating the fluorite from the gangue in froth-flotation cells.

The slurried mix of the fluorite and gangue, quebracho, and water is mixed with a collector which can be a fatty acid, such as oleic acid and the like. The slurry then formed consists of 2.2 to 2.8 pounds of quebracho, and 4.3 to 4.7 pounds of the fatty acid per ton of dry ore.

In most cases, where the gangue is harder than the minerals, dry sequential impact grinding preferentially reduces the weaker material in size leaving a coarse barren gangue. If desired, the coarse barren gangue can be separated from the fine-sized minerals by conventional means, such as screening, prior to froth-flotation. If the mineral, in this case, fluorite, is the material being floated and the coarse gangue is not removed by screening, the gangue will sink quickly in the first series or rougher cells in the froth-flotation step and can be removed as tailings. The float product from the rougher cells is sequentially passed to cleaner cells. The mineral (fluorite) is recovered from the cleaner cells as a float product, and the coarse and fine gangue settle to form a sink product which is passed to the scavenger cells and then to waste.

It must be understood that the above specific example of processing fluorite is given only as an illustration of the process of the invention and is not to be interpreted as limiting the process of the invention to any specific number of dry centrifugal impact grinding mills nor to any specific peripheral speeds at which the rotors in the dry centrifugal impact grinding mills are operated since all ores have different fracture characteristics which will determine the number of dry centrifugal impact grinding mills needed and the varying speeds at which the rotors must be operated in order to obtain comminution of the ores and liberation of minerals from gangue at a specific liberation size to produce a predetermined grade of concentrate of the particular mineral which is being processed. The fracture characteristics of ores can be determined on a laboratory basis prior to comminution by the process described herein.

It will be understood that any percentages mentioned in these specifications and claims are on a weight basis unless otherwise described.

We claim:

1. A method for preparing an ore containing minerals and gangue for separation of said minerals from said gangue and recovery of said minerals as a concentrate of a predetermined grade, said recovery being made at a relatively coarse liberation size, particles of said ore being crushed to a desired size prior to preparation, said method comprising:

a. sizing said crushed ore at a predetermined size to separate said crushed ore into an oversize fraction and an undersize fraction,

b. recycling said oversize fraction to said crushing step,

c. passing said undersize fraction sequentially to a plurality of dry centrifugal impact grinding mills, each of said mills having a rotor which is operated at a predetermined speed to hurl said crushed ore against a target at a velocity whereby said crushed ore attains an impact energy of sufficient magnitude to liberate said minerals from said gangue, said liberation being at a relatively coarse size,

d. collecting said liberated minerals and said gangue as a comminuted product,

e. passing said comminuted product to mineral separation and recovery steps wherein said minerals are separated from said gangue and are recovered as a concentrate of a predetermined grade.

2. The method of claim 1 wherein the size separation of step (a) is made at one-half of an inch.

3. The method of claim 1 wherein the ore to be prepared is at least one ore taken from the group consisting of fluorite ore and iron ore.

4. The method of claim 1 wherein the ore treated is fluorite ore.

5. A method for preparing a fluorite ore containing fluorite and gangue for separation of said fluorite from said gangue and recovery of said fluorite as a concentrate containing not less than fluorite, said recovery being made at a liberation size of -l00 mesh size, said fluorite ore being crushed to about one-half of an inch, said method comprising:

a. sizing said crushed fluorite ore at /:z of an inch size to separate said crushed fluorite ore into a /2 of an inch size fraction and a /z of an inch size fraction,

b. recycling said /2 of an inch size fraction to said crushing step,

c. passing said %a of an inch size fraction sequentially to a plurality of dry centrifugal air impact grinding mills each having a rotor operated at a predetermined speed to liberate said fluorite from said gangue at a liberation size of mesh size,

. collecting said liberated fluorite and said gangue as a comminuted product,

e. passing said comminuted product to mineral separation and recovery steps wherein said fluorite is separated from said gangue and is recovered as a concentrate containing not less than 95% fluorite and not more than 1.0% phosphorus.

6. The method of claim 5 wherein the /2 of an inch size fraction is comminuted in step (c) by being passed sequentially to a first dry centrifugal air impact grinding mill having a rotor operating at a peripheral speed of about 175 feet per second, to a second dry centrifugal air impact grinding mill having a rotor operating at a peripheral speed of about 250 feet per second to liberate a portion of the fluorite from gangue, sizing the product of the second dry centrifugal air impact grinding mill on a 100 mesh sieve to form a first +100 mesh size fraction, passing said first 30 100 mesh size fraction to a third dry centrifugal air impact grinding mill having a rotor operating at a peripheral spped of about 300 feet per second to liberate a second portion of the fluorite from gangue, sizing the product of said third dry centrifugal air impact grinding step at 100 mesh size to obtain a second +100 mesh size and a second -l mesh size, passing said second +100 mesh size to a fourth dry centrifugal air impact grinding mill having a rotor operating at a peripheral speed of 400 feet per second to liberate a third portion of said fluorite from said gangue, sizing the product fromsaid fourth dry centrifugal air impact grinding mill at 70 mesh size to produce a mesh size and a -70 mesh size, passing said +70 mesh size to waste and combining the first and second 1OO mesh sizes and the -70 mesh size.

7. The method of claim 6 wherein the mineral separation and recovery steps of step (e) comprise:

f. mixing the undersize liberated fluorite and gangue from the sizing steps with a predetermined amount of quebracho and collector,

g. passing said mixture to a series of froth-flotation cells to separate the fluorite as a float product from the gangue as a sink product,

h. recovering said fluorite in said float product as a usable concentrate of a predetermined grade, and

i. passing the gangue as a sink product to waste.

8. The method of claim 7 wherein the amount of quebracho added in step (f) is about 2.2 pounds to about 2.8 pounds per ton of comminuted product and the amount of a collector is about 4.3 pounds to about 4.7

pounds per ton of comminuted product. 

1. A METHOD FOR PREPARING AN ORE CONTAINING MINERALS AND GANGUE FOR SEPERATION OF SAID MINERALS FROM SAID GANGUE AND RECOVERY OF SAID MINERALS AS A CONCENTRATE OF A PREDETERMINED GRADE, AND RECOVERY BEING MADE AT A RELATIVELY COARSE LIBERATION SIZE, PARTICLES OF SAID ORE BEING CRUSHED TO A DESIRED SIZE PRIOR TO PREPARATION, SAID METHOD COMPRISING: A. SIZING SAID CRUSHED ORE AT A PREDETERMINED SIZE TO SEPERATE SAID CRUSHED ORE INTO AN OVERSIZE FRACTION AND AN UNDERSIZE FRACTION, B. RECYCLING SAID OVERSIZE FRACTION TO SAID CRUSHING STEP, C. PASSING SAID UNDERSIZE FRACTION SEQUENTIALLY TO A PLURALITY OF DRY CENTRIFUGAL IMPACT GRINDING MILLS, EACH OF SAID MILLS HAVING A ROTOR WHICH IS OPERATED AT A PREDETERMINED SPACED TO HURL SAIID CRUSHED ORE AGAINST A TARGET AT A VELOCITY WHEREBY SAID CRUSHED ORE ATTAINS AN IMPACT ENERGY OF SUFFICIENT MAGNITUDE TO LIBERATE SAID MINERALS FROM SAID GANGUE, SAID LIBERATION BEING AT A RELATIVELY COARSE SIZE, D. COLLECRTING SAID LIBERATED MINERALS AND SAID GANFUE AS A COMMINUTED PRODUCT, E. PASSING SAID COMMINUTED PRODUCT TO MINERAL SEPARATION AND RECOVERY STEPS WHEREIN SAID MINERALS ARE SEPARATED FROM SAID GANGUE AND ARE RECOVERED AS A CONCENTRATE OF A PREDETERMINED GRADE.
 2. The method of claim 1 wherein the size separation of step (a) is made at one-Half of an inch.
 3. The method of claim 1 wherein the ore to be prepared is at least one ore taken from the group consisting of fluorite ore and iron ore.
 4. The method of claim 1 wherein the ore treated is fluorite ore.
 5. A method for preparing a fluorite ore containing fluorite and gangue for separation of said fluorite from said gangue and recovery of said fluorite as a concentrate containing not less than 95% fluorite, said recovery being made at a liberation size of -100 mesh size, said fluorite ore being crushed to about one-half of an inch, said method comprising: a. sizing said crushed fluorite ore at 1/2 of an inch size to separate said crushed fluorite ore into a + 1/2 of an inch size fraction and a - 1/2 of an inch size fraction, b. recycling said + 1/2 of an inch size fraction to said crushing step, c. passing said - 1/2 of an inch size fraction sequentially to a plurality of dry centrifugal air impact grinding mills each having a rotor operated at a predetermined speed to liberate said fluorite from said gangue at a liberation size of 100 mesh size, d. collecting said liberated fluorite and said gangue as a comminuted product, e. passing said comminuted product to mineral separation and recovery steps wherein said fluorite is separated from said gangue and is recovered as a concentrate containing not less than 95% fluorite and not more than 1.0% phosphorus.
 6. The method of claim 5 wherein the - 1/2 of an inch size fraction is comminuted in step (c) by being passed sequentially to a first dry centrifugal air impact grinding mill having a rotor operating at a peripheral speed of about 175 feet per second, to a second dry centrifugal air impact grinding mill having a rotor operating at a peripheral speed of about 250 feet per second to liberate a portion of the fluorite from gangue, sizing the product of the second dry centrifugal air impact grinding mill on a 100 mesh sieve to form a first +100 mesh size fraction, passing said first 30 100 mesh size fraction to a third dry centrifugal air impact grinding mill having a rotor operating at a peripheral spped of about 300 feet per second to liberate a second portion of the fluorite from gangue, sizing the product of said third dry centrifugal air impact grinding step at 100 mesh size to obtain a second +100 mesh size and a second -100 mesh size, passing said second +100 mesh size to a fourth dry centrifugal air impact grinding mill having a rotor operating at a peripheral speed of 400 feet per second to liberate a third portion of said fluorite from said gangue, sizing the product from said fourth dry centrifugal air impact grinding mill at 70 mesh size to produce a +70 mesh size and a -70 mesh size, passing said +70 mesh size to waste and combining the first and second -100 mesh sizes and the -70 mesh size.
 7. The method of claim 6 wherein the mineral separation and recovery steps of step (e) comprise: f. mixing the undersize liberated fluorite and gangue from the sizing steps with a predetermined amount of quebracho and collector, g. passing said mixture to a series of froth-flotation cells to separate the fluorite as a float product from the gangue as a sink product, h. recovering said fluorite in said float product as a usable concentrate of a predetermined grade, and i. passing the gangue as a sink product to waste.
 8. The method of claim 7 wherein the amount of quebracho added in step (f) is about 2.2 pounds to about 2.8 pounds per ton of comminuted product and the amount of a collector is about 4.3 pounds to about 4.7 pounds per ton of comminuted product. 